Oddities in the period of Titan's rotation may imply that the surface of the …

Almost any description of research involving Saturn's moon Titan can be challenging to introduce because there are simply so many things about the moon that defy what we've come to consider normal from an earth-bound perspective. For starters, tidal forces generated by presence of the massive Saturn have synchronized Titan's rotation with its orbit, so that both take 16 days. Titan's seasonal cycle is also controlled by Saturn's movement around the sun, meaning it lasts 20 years.

Oddities go well beyond its orbit, however. Titan's atmosphere is an incredibly dense mix of hydrocarbons that prevent observations of its surface with visible light; NASA's Cassini mission has to map surface features using radar. Those maps have revealed a bizarro-world landscape filled with earth-like features made of completely different materials. Mountains, lakes, oceans, and dunes all exist on Titan, but are composed of various forms of solid and liquid hydrocarbons. Believe it or not, all of these features come into play if you hope to understand a paper that will be showing up in Science later today.

The paper's foundation is two different sets of radar mapping data from Cassini, one made in 2004, the second two and a half years afterwards. If we know where a feature, such as a mountain range or lake, is and know the rotation of Titan, then we should be able to predict where that feature was going to be in the later set of observations. Oddly, this didn't work. Some features, in fact, were as much as 30 km from where we'd predict they'd be seen.

It's technically possible that there's something missing in our understanding of the basics of Titan's rotation, but the authors of the new paper suggest an intriguing explanation: the rotation of the moon's surface isn't entirely synchronized with the rest of the moon. Various calculations suggested that Titan could have a frozen crust on top of a liquid water or water/ammonia ocean, much like what has been found on some of Jupiter's moons. If the crust is thin enough, and the ocean deep enough, this could partly decouple the crust's rotation from that of the moon's rocky core.

This would still require something to push Titan's surface out of synch with its ocean and core, but the authors have an explanation: seasonal changes in winds. These forces actually alter Earth's day length by up to a millisecond, but the vast mass differences between the earth and air keep the impact negligible. On Titan, the extremely dense atmosphere could, when combined with a thin crust, create more significant variability. The authors calculate that the moment of inertia of the crust may be as little as 400 times larger than that of the atmosphere.

If the Cassini mission is extended out to 2010, further radar mapping should help distinguish between the two alternative explanations for this discrepancy (or maybe even suggest a third). In the mean time, it's worth pondering how much stranger still this makes Titan. It's possible that, in addition to the strange yet familiar hydrocarbon world of the surface, Titan has a world of Europa-style liquid oceans hidden a 100 kilometers down.